The present invention relates to subterranean operations and, more particularly, to settable drilling fluids that comprise an aqueous-based drilling fluid and cement kiln dust (“CKD”), and associated methods of use in subterranean operations.
During the drilling of a well bore in a subterranean formation, a drilling fluid may be used to, among other things, cool the drill bit, lubricate the rotating drill string to prevent it from sticking to the walls of the well bore, prevent blowouts by serving as a hydrostatic head to counteract the sudden entrance into the well bore of high pressure formation fluids, and remove drill cuttings from the well bore. A drilling fluid may be circulated downwardly through a drill pipe and drill bit and then upwardly through the well bore to the surface. A variety of drilling fluids may be used during the drilling of well bores into a subterranean formation. For instance, the drilling fluid used may be any number of fluids (gaseous or liquid) and mixtures of fluids and solids (such as solid suspensions, mixtures, and emulsions).
Once the well bore has been drilled to a desired depth, the drill string and drill bit may be removed from the well bore and the drilling fluid may be left in the well bore to provide hydrostatic pressure on the formation penetrated by the well bore, e.g., to prevent the flow of formation fluids into the well bore. Next, a pipe string (e.g., casing, liners, etc.) may be introduced into the well bore. Depending on the depth of the well bore and whether or not any problems are encountered in introducing the pipe string into the well bore, the drilling fluid may remain relatively static in the well bore for a relatively long time period, for example, up to 24 hours or longer. While drilling fluids are not settable (e.g., they generally do not to form a hardened mass over time), drilling fluids may increase in gel strength over time. As referred to herein, “gel strength” is the measure of the ability of a colloidal dispersion to form a gel and is based upon its resistance to shear. Accordingly, during the time period that the drilling fluid is static in the well bore, portions of the drilling fluid may increase in gel strength so that displacement of the drilling fluid from within the well bore may become more difficult.
After the pipe string has been introduced into the well bore, the pipe string may be cemented in the well bore by pumping a cement composition into an annular space between the walls of the well bore and the pipe string disposed therein, thereby displacing the drilling fluid in the annulus. However, if the drilling fluid has developed sufficient gel strength due to remaining static in the well bore, portions of the drilling fluid in the annulus may not be displaced. Since the drilling fluid generally is not settable, this may be problematic. For example, because the drilling fluid may remain on the surfaces of the subterranean formation and on the surfaces of the pipe string, a suitable bond between the cement composition and the surfaces may not result. This may lead to the loss of zonal isolation, which can result in formation fluids continuing to enter and flow in the well bore.
In order to solve the above-described problem, settable drilling fluids have been used heretofore. Generally, a settable drilling fluid should be formulated so that it is compatible with drilling fluid (if any) remaining in the well bore. In some instances, the settable drilling may be formulated using at least a portion of the drilling fluid that was used in the drilling of the well bore. Because the settable drilling fluid should be compatible with any fluids remaining in the well bore, use of a settable drilling fluid in subsequent cement operations (e.g., primary cementing) may reduce the problems associated with incompatibility between a cement composition and the drilling fluid remaining in the well bore. Additionally, use of a settable drilling fluid may alleviate problems associated with disposal of the used drilling fluid.
During the manufacture of cement, a waste material commonly referred to as “CKD” is generated. “CKD,” as that term is used herein, refers to a partially calcined kiln feed that is typically removed from the gas stream and collected during the manufacture of cement. Usually, large quantities of CKD are collected in the production of cement, and they are commonly disposed of as waste. Disposal of the waste CKD can add undesirable costs to the manufacture of the cement, as well as the environmental concerns associated with its disposal. The chemical analysis of CKD from various cement manufactures varies depending on a number of factors, including the particular kiln feed, the efficiencies of the cement production operation, and the associated dust collection systems. CKD generally may comprise a variety of oxides, such as SiO2, Al2O3, Fe2O3, CaO, MgO, SO3, Na2O, and K2O.